Ultra Wideband (UWB) technology, which is useful for both communication and sensing applications, is based on very short pulses and time domain signal processing. A very commonly used pulse in UWB systems is the monocycle and as the monocycle's width determines the bandwidth, a narrow pulse width is necessary for producing an ultra wideband signal.
There are several methods of generating pulses and devices used for pulse generation include, for example, tunnel diodes, avalanche transistors, and step recovery diodes (SRDs). In Ultra Wideband (UWB) applications, each pulse may represent a symbol. In a typical UWB application, the pulses are followed by a silence period (a space). The characteristics of the pulse are changed to represent the data.
FIG. 1 shows conventional pulse position modulation where the position of the pulse is either advanced or delayed from its mean position to represent a symbol. FIG. 2 shows conventional bi-phase modulation of the pulse to represent the symbol. In FIGS. 1 and 2, the distance between the peaks of the waveforms represents the pulse repetition period.
For high data rate applications, it is imperative that the pulse width is low to permit more pulses to be transmitted in a given period. If only one cycle of a pulse is generated, the energy may be spread over a wide frequency band. Also, the data rate may be improved as the silence period is larger and so more pulses may be transmitted, for a given duty cycle, by multiplexing other channels.
One conventional way of generating very narrow pulses is to use Step Recovery Diodes (SRDs).
Although there are many fast square wave pulse generators commercially available, there are few high speed monocycle generators.
Monocycles may be generated by twice differentiating the rising edge and falling edge of square pulses using differentiators or Impulse Forming Networks. This is described in the Impulse Forming Networks Data Sheet of Picosecond Pulse Labs. This document describes the use of the differentiation of fast rise time signals to generate pulses. Differentiation of the leading edge produces a positive impulse and differentiation of a trailing edge produces a negative impulse. One more differentiation produces a monocycle. Whilst passive resistor and capacitor elements may be used for the differentiation, the amplitude and the pulse width of the resultant monocycle depends, to a large extent, on the rise time and the fall time of the signal.
There are a number of further problems with this approach. Firstly, circuits for generating signals with fast rising edges with rise times of the order of tens of picoseconds are needed and such circuits or commercial instruments are generally expensive and not economical for low cost applications. Secondly, for every monocycle generated by the rising edge, a 180 degrees phase shifted monocycle would be generated by the falling edge. This reduces the flexibility of this approach. Thus the generation of sub-nanosecond monocycle pulses with pulse repetition rates of up to 1 GHz using low cost circuitry is very desirable. Most conventional monocycle generators use lumped elements instead of distributed elements and thus are more expensive and less repeatable due to component tolerances.
A number of alternative conventional monocycle generators use several active devices in the circuit. For example, in the system described in the document by Jeong Soo Lee, Cam Nguyen and Tom Scullion entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, Step Recovery Diodes (SRDs) are used together with Schottky diodes for generating very narrow pulses. The Schottky diode is included to overcome the ringing effect which tends otherwise to be exhibited as narrower monocycles and higher pulse repetition rates are attempted in systems using SRD circuits for generating sub-nanosecond monocycles.
The method described in the document by Jeong Soo Lee, Cam Nguyen and Tom Scullion in the document entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, combines two Gaussian pulses to produce a monocycle. The two Gaussian pulses are 180 degrees out of phase and have a time delay between them.
FIG. 3 shows the circuit for generating a monocycle according to the above mentioned publication by Jeong Soo Lee, Cam Nguyen and Tom Scullion. The circuit is driven by a local oscillator 1 which supplies 10 MHz square wave signal to the anode of an SRD diode 2. The cathode of the SRD diode is connected to a 50 Ohm short circuited transmission line 3 and to the anode of a Schottky diode 4. The cathode of the Schottky diode 4 is connected to a capacitor 6 and to a resistor 8. The resistor 8 is earthed. The capacitor 6 is connected to two further transmission lines 10, 12, one of which is terminated 12 and the other of which is short circuited 10.
This method has the disadvantage of wider pulse width, as the width of the monocycle is twice the impulse width. Furthermore, the use of Schottky diodes to limit the ringing effect adds to the cost of the pulse generator.
The document by Jeong Soo Lee, Cam Nguyen and Tom Scullion in the document entitled “New Uniplanar Subnanosecond Monocycle Pulse Generator and Transformer for Time-Domain Microwave Applications”, June 2001 IEEE Transactions On Microwave Theory And Techniques, Vol. 49, No. 6, pp 1126-1129, also describes a pulse-to-monocycle converter. This differs from the circuit described above in that the SRD 2 is omitted, together with the short circuited transmission line 3. However, the converter requires a narrow pulse to drive it instead of a square wave and it is not itself a pulse generator.
In another prior art document, entitled “A New Ultra-Wideband, Ultra-Short Monocycle Pulse Generator With Reduced Ringing”, Jeongwoo Han and Cam Nguyen, June 2002, IEEE Microwave And Wireless Components Letters, Vol. 12, No. 6, pp 206-208, a system is described and which is illustrated in FIG. 4. The circuit includes a square wave generator 14 which is connected to the anode of an SRD 16. The cathode of the SRD is connected to a short circuited transmission line 18 and also to the anode of a Schottky diode 20. The cathode of the Schottky diode is connected to a terminated transmission line 22 and to a capacitor 24. The output of the capacitor 24 is connected to the cathode of a further Schottky diode 26, the anode of which is earthed. The output of the capacitor 24 is also connected to a resistor 28 and to a further capacitor 30, the output of which is earthed by a further resistor 32. The output of the resistor 28 is connected, via a further capacitor 34, to ground. A voltage source 36 is connected across the capacitor 34. The SRD 16 produces a Gaussian pulse and the resistor 32 and capacitor 30 form a high pass filter which acts as a differentiator to convert the Gaussian pulse into a monocycle. The width of the monocycle formed after differentiation of the Gaussian pulse is almost the same as that of the pulse itself. The two Schottky diodes 20 and 26 act to reduce the ringing effect. The main disadvantage associated with this system is the use of the Schottky diodes, which adds to the cost of the system.
In Jeong Soo Lee and Cam Nguyen, “Novel Low-cost Ultra-Wideband, Ultra-Short-Pulse Transmitter with MESFET Impulse-Shaping Circuitry for Reduced Distortion and Improved Pulse Repetition Rate”, May 2001, IEEE Microwave And Wireless Components Letters, Vol. 11, No. 5, pp 208-210, a system is described which includes, as shown in FIG. 5, a generator 37 connected to the cathode of an SRD 38, the anode of which is connected to a short circuited transmission line 40. The anode of the SRD 38 is also connected to an earthed resistor 42 and to the gate of a MESFET 44. The source of the MESFET 44 is earthed. The drain of the MESFET 44 is connected to the anode of a Schottky diode 46. The cathode of the Schottky diode 46 is connected to an earthed resistor 48 and also to a capacitor 50. The output of the capacitor 50 is connected to a short circuited transmission line 52 and to the input of an MMIC amplifier 54. The output of the MMIC amplifier 54 is terminated in a resistor 56 which is connected to ground. The MESFET 44 is used as an impulse-shaping network and it enables the circuit to achieve higher pulse repetition frequencies of up to several hundreds of mega Hertz. However, the use of the MESFET 44, and the Schottky diode 46 add to the cost of the system.
U.S. Pat. No. 4,442,362 describes a short pulse generator using an SRD. A plurality of capacitors are charged in parallel and then connected in series by a plurality of avalanche transistors to obtain a voltage which is substantially equal to the sum of the capacitor voltages when charged. The series coupled capacitors are then coupled, via an output avalanche transistor, to a differentiator which produces a monocycle pulse. This method can generate high peak amplitude pulses. However, the use of the avalanche transistors adds to the cost of the system making it too expensive for low cost systems. Also, the use of avalanche transistors limits the pulse repetition rate.
U.S. Pat. No. 3,622,808 describes a pulse shaping circuit for producing high frequency pulses using two step recovery diodes and other lumped components. The circuit is shown in FIG. 6. A signal source 58, for example, a sine wave, is connected to an inductance 60, the other end of which is connected via a resistor 62 to a voltage source (not shown). The signal source 58 is also connected to the cathode of an SRD 64. The anode of the SRD 64 is connected via a further inductor 66 to ground. The anode of the SRD 64 is also connected to the cathode of a further SRD 68 and to the output 70 of the system. The anode of the further SRD 68 is connected via a capacitor 72 to ground and via a resistor 74 to a further power supply (not shown). This system produces narrow pulses at high frequency but does not itself produce a monocycle. The main disadvantage of this system is the high cost of the system.
Thus, there is a need for a low cost monocycle generator preferably capable of generating sub-nanosecond monocycles with pulse repetition frequencies in excess of 1 GHz.